This invention relates to processes for recovery and purification of silica.
There is a current need for pure silica. Both high-purity silica, used as a filler for plastic-moulded semi-conductor devices, and high-purity silicon, used in solar cells and silicon chips, are required for diverse industrial applications. It would also be useful to be able to recover and selectively remove silicate from aqueous media (for example, silicon isotope recovery), particularly when present in low concentrations. Traditionally silica has been recovered from aqueous silicate solutions at low pH by precipitation. Procedures for purification of silicon or silica include:
(i) powdering and acid-leaching of metallurgical grade silicon followed by passage through an induction plasma. On solidification impurities migrate to the surface of granules and can be leached;
(ii) acid leaching of metallurgical silicon followed by chlorination at 1500.degree. C. and final zone refining;
(iii) vacuum refining at 1500.degree. C. and fluxing at 1410.degree. C. with the fluoride of an alkaline earth or alkali metal;
(iv) aluminothermic reduction of quartz sand in a calcium silicate slag at 1550.degree. C.
(v) carbothermic reduction of silica in an electric arc; or
(vi) pyrolysis of silicon hydride.
Traditional silica precipitation methods are not well suited to the recovery of low concentrations of soluble silicate. Most purification methods suffer from a lack of selectivity in element removal, and involve time-consuming and expensive multistep procedures.
The formation of silicomolydbates has been reported by Truesdale and Smith in The Analyst, 101; 19 (1976). Precipitation of silicomolybdates by organic bases for analytical purposes has been described by MacDonald et al in The Analyst, 93, 65 (1968) and Piryutko et al in Chem. Abs. 95, 143516v. Tetraalkylammonium silicomolybdates are discussed in Filowitz et al, J. Amer. Chem. Soc. 98, 2345 (1976).